1. Field of the Invention
The present invention relates to a driving circuit employing a synchronous rectifier circuit in which the primary winding side and the secondary winding side of a transformer are insulated from each other.
2. Description of the Related Art
FIG. 2 shows a conventional synchronous rectifier circuit of a forward converter. In the figure, reference numeral 1 indicates a transformer, reference numeral 2 indicates a main switch, reference numeral 3 indicates a rectifying FET (field effect transistor), reference numeral 4 indicates a commutating FET, reference numerals 5 and 6 indicate drive resistors, reference numeral 7 indicates an output choke (coil), and reference numeral 8 indicates a smoothing capacitor.
In this circuit, (i) a series circuit consisting of the input source and the main switch 2 is connected to the primary winding of the transformer 1, (ii) a series circuit consisting of the output choke 7 and the load is connected to the commutating FET 4 in parallel, (iii) an end of the commutating FET 4 is connected to an end of the secondary winding of the transformer 1 in series, and the other end of the commutating FET 4 is connected to the other end of the secondary winding via the rectifying FET 3, (iv) the gate of the rectifying FET 3 is connected to the drive resistor 5, which is connected to a connecting point provided between the secondary winding and the output choke 7, (v) the gate of the commutating FET 4 is connected to the drive resistor 6, which is connected to the other end of the secondary winding of the transformer 1, and (vi) the smoothing capacitor 8 is connected to the load in parallel.
The operation of the synchronous rectifier circuit having the above-explained structure will be explained below. When the main switch 2 is turned off, a voltage is generated between the drain and the source of the main switch 2, thereby inverting the voltage of the secondary winding. In this process, the induced voltage at the secondary winding is applied via the drive resistor 6 to the commutating FET 4, thereby turning on the commutating FET 4.
In this synchronous rectifier circuit, if voltage remains at the output of the secondary (winding) side when the driving pulse stops at the primary (winding) side, then a voltage is generated between the drain and the source of the commutating FET 4, and simultaneously, a voltage is also applied to the gate of the rectifying FET 3. Accordingly, a reverse current starts to flow from the output choke 7 via the transformer 1 and the rectifying FET 3, and the voltage generated by this reverse current and the excitation of the output choke 7 may damage elements in the circuit.
In order to solve the above problem, as shown in FIG. 3, a diode 14 is inserted between the gate and the source of the rectifying FET 3 and a capacitor 10 is connected to the gate of the rectifying FET 3, so as to simply divide the output of the transformer. Such a driving method is generally used. However, in this method, when the primary side is stopped and the commutating FET 4 is shifted from the ON to the OFF state (i.e., turned off from the ON state), a voltage is generated between the terminals of the output choke which is excited by the current drawn from the output side of the synchronous rectifier circuit, and simultaneously, a voltage is generated between the primary and secondary windings of the transformer 1. The rectifying FET 3 turns on immediately after the voltage is generated between the primary and secondary windings, so that the reverse current is easily produced.